A wide variety of substances have been proposed for use as fat substitutes in food compositions. The chemical structures of such substances are selected such that they are more resistant to breakdown by the metabolic processes of the human digestive system which normally occur upon ingestion of conventional triglyceride lipids. Because of their increased resistance to digestion and absorption, the number of calories per gram available from these fat substitutes is considerably reduced as compared to common vegetable oils, animal fats, and other lipids. Such substances thus may be utilized in the preparation of reduced calorie food compositions useful in the control of body weight.
U.S. Pat. No. 4,861,613 describes one class of particularly useful fat substitutes wherein a polyol such as glycerin is alkoxylated with an epoxide such as propylene oxide under basic conditions and then esterified with any of a number of fatty acids to form an esterified alkoxylated polyol. These substances have the physical and organoleptic properties of conventional triglyceride lipids, yet are significantly lower in available calories owing to their pronounced resistance towards pancreatic lipase enzymatic hydrolysis. The thermal and oxidative stability of the esterified alkoxylated polyols renders them especially suitable for use in the preparation of reduced calorie food compositions requiring exposure to high temperatures.
Unfortunately, as a consequence of their hydrolytic stability, low digestibility, and lipophilic character the esterified alkoxylated polyols described in U.S. Pat. No. 4,861,613 which are fully liquid at body temperature may tend to cause certain undesirable gastrointestinal side effects when consumed at high levels in the diet. That is, since such esterified alkoxylated polyols are not readily broken down into simpler substances upon ingestion, they retain their oily, fat-like character and pass through the digestive tract in substantially unaltered form. Problems with leakage of the fat substitute through the anal sphincter and separation of the fat substitute as an oil from the excreted fecal matter can occur as a result of the non-digestibility of the fat substitute. Other fat substitutes which are similarly resistant towards digestion are known to produce the same sort of gastrointestinal side effects. Examples include sucrose polyester which is esterified with up to 8 fatty acid groups; see U.S. Pat. Nos. 3,954,976, 4,005,195, 4,005,196, and 5,006,360. Obviously, such problems will greatly limit the maximum level of these substances which can be tolerated in various food compositions, thereby constraining the amount of conventional triglyceride and the number of calories which can be removed from certain foods.
Despite the considerable research performed in the last two decades on synthetically prepared fat substitutes, an understanding of the precise relationship between chemical structure and digestability is still lacking and the field remains a highly uncertain and unpredictable art. The technical literature related to fat substitutes is replete with conflicting observations and findings which cannot easily be reconciled or explained. For example, U.S. Pat. No. 4,861,613 (White et al.) teaches that a polyol such as glycerin should be reacted (epoxylated) with a quantity of a C.sub.3 -C.sub.6 epoxide sufficient to convert greater than 95% of the primary hydroxyl groups of the polyol to secondary or tertiary hydroxyl groups prior to esterification with a fatty acid in order to obtain a low calorie fat substitute. The non-digestibility of the final esterified alkoxylated polyol was attributed primarily to the presence of secondary and tertiary ester linkages since substances with lower degrees of alkoxylation were found to be susceptible to lipase-catalyzed hydrolysis.
In contrast, U.S. Pat. No. 4,849,242 (Kershner) teaches the preparation of reduced calorie food compositions containing oil-like polymer fatty acid esters having the property of being substantially hydrolyzed during the process of intestinal digestion into a mixture of fatty acids and a non-caloric water-soluble or water-dispersible polymeric alcohol. Fatty acid esters of water-soluble polyoxyalkylenes are said to be particularly useful for this purpose. Kershner teaches that polyoxyethylenes, polyoxypropylenes, and polyoxybutylenes are all equally well-suited for use as the polyoxyalkylene starting material, thus implying that the fatty acid esters of such substances will all be readily hydrolyzed upon ingestion. Thus, no distinction between primary and secondary ester linkages in terms of their susceptibility to enzyme-catalyzed hydrolysis was recognized.
Quite different conclusions were reached in U.S. Pat. Nos. 5,059,443 (Ennis et al.) and 5,077,073 (Ennis et al.), which respectively describe the use of esterified alkoxylated alkyl glycosides and esterified alkoxylated sugars and sugar alcohols as low calorie fat substitutes. The degree and rate of hydrolysis of the ester bonds were found to be quite low for these substances relative to a conventional triglyceride. Moreover, the resistance to hydrolysis was reported to be approximately equally high regardless of whether ethylene oxide or propylene oxide was utilized in the alkoxylation. That is, no significant difference in reactivity was observed between substances with primary ester linkages (derived from ethylene oxide) and substances with secondary ester linkages (derived from propylene oxide).